1. Field of the Invention
The invention relates to the field of current-driven electronic devices such as visual display devices. More particularly, the invention relates to current balancing circuits for devices requiring accurate, matched and repeatable current drivers, for example visual displays having arrays of light-emitting sources.
2. Description of the Related Technology
Visual display devices are widely used to present visual information and cues to users, operators or viewers of various systems. Not infrequently, visual displays use arrays of light-emitting sources, often consisting of diodes organized in a columnar configuration. These arrays are often arranged such that columns of light-emitting sources are driven by individual current sources. These light-emitting sources are also commonly connected to externally switched rows to complete the electrical circuit, thereby allowing proper illumination of the visual display.
As visual displays typically consist of a multitude of these arrays of light-emitting sources, several (for example 3–4) integrated electronic circuits are required to connect all the columns. Physically, these integrated circuits are necessarily very long and narrow to accommodate the large number of connections and to match the linear connection arrangement of the array. This wide physical separation of circuit components permits temperature variations between sensitive elements, often resulting in performance variations among these elements. In addition, variations in the manufactured characteristics of electronic components also often result in unpredictable and varying performance. Such performance variations often cause poor matching of the current sources at the ends of these individual integrated circuits. When the currents at the ends of an individual column driver circuit are not well matched, the result is a variation in brightness at these end columns that make it difficult to match them to the adjacent columns driven by separate driver circuits. This abrupt discontinuity in brightness is often noticeable to the users of the visual display devices.
Typically, manufacturers in the industry of visual display devices attempt to match all adjacent columns in the same integrated circuit. As the electronic components for adjacent columns are typically located in close proximity on the electronic circuit layout, they tend to be inherently closely matched. In addition, as the eye is relatively insensitive to slowly changing spatial brightness, it is not particularly essential that all adjacent columns of light-emitting sources within an individual integrated circuit be absolutely uniform provided that the differences are not abrupt.
However, when there is a difference in the current sources, a discontinuity often results between columns. As the human eye is very discerning of differences in brightness at sharp edges of light patterns, this results in a noticeable discontinuity in the smoothness of the visual display, resulting in a perceptible degradation in the quality of the display. Accordingly, there is a need in the technology for a column driver circuit in which current sources are closely matched.